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python/s/deep-flatten.md

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+---
+title: Deep flatten list
+type: snippet
+language: python
+tags: [list,recursion]
+cover: mask-quiet
+dateModified: 2020-12-29T19:53:45+02:00
+---
+
+Deep flattens a list.
+
+- Use recursion.
+- Use `isinstance()` with `collections.abc.Iterable` to check if an element is iterable.
+- If it is iterable, apply `deep_flatten()` recursively, otherwise return `[lst]`.
+
+```py
+from collections.abc import Iterable
+
+def deep_flatten(lst):
+  return ([a for i in lst for a in
+          deep_flatten(i)] if isinstance(lst, Iterable) else [lst])
+```
+
+```py
+deep_flatten([1, [2], [[3], 4], 5]) # [1, 2, 3, 4, 5]
+```